1. Field of the Invention
The present invention relates generally to brushless and sensorless DC motors, and in particular, to a method and apparatus for aligning a rotor of the motor with respect to a stator of the motor before accelerating the motor to an operating rotation speed.
2. Description of Related Art
Brushless and sensorless DC motors are often started using an "align and go" technique. During the align step, the rotor of the motor is rotated to a known position relative to the stator. Thereafter, the rotor is accelerated to a predetermined rotation speed and controlled to rotate at a desired rotation speed.
Conventional motors of this type contain a stator having three drive coils arranged in a "Y" configuration, each coil being arranged to be phase separated from each other by 120.degree. as shown in FIG. 1. To align the rotor with respect to the stator, a current is passed through two of the coils (e.g., A and B) while the remaining terminal (e.g., C) remains open circuited with respect to the center tap (e.g., T). Under the influence of the current applied to the two coils, the rotor rotates to a rotary position where the magnetic flux generated by the current in the two energized coils is maximized. The exact rotary motion of the rotor depends on the drive current and the rotor's inertia and damping characteristics; however, after a predetermined worst case settling time interval, the rotor is aligned based on the selected and energized coils.
This technique is sufficient when applied to an ideal frictionless motor. However, real motors are characterized by friction: a dynamic friction which becomes part of the damping characteristics and a static friction which is ordinarily greater than the dynamic friction and which may prevent the rotor from moving at all when the torque applied is small. Typically, the torque applied to the rotor of the motor depends on the rotor angle with respect to the energized coils. If the torque is insufficient to overcome the static friction, the rotor does not properly align.